carlin



Aug. 11, 1959 u H. J. CARLIN I I MULTIPLEX SIGNALLING SYSTEM Filed June22, 1955 I t 2 Sheets-Sheet 1 Tunable Detector Resonontcovitytuninq |bIn accomplished by this sliding section H "In" 3 4\\\\\\\ \2\ fill/III dII/I/l/A b c c A (next coupler) 0 i r. Antenna Resonant fl y 90 H 9 T V2 S Anti-resonant (transmitter 7c" cavity Impedance or receiver) q Melcomm 7d 4 Equivalent circuit 6b Mfla.

(T) Trunsmitter' 6c Recelyer INVENTOR HERBERT J. CARLIN' ATTORNEY-11,1959 H. J. CARLIN 2,899,650

v MULTIPLEX SIGNALLING SYSTEM Filed June 22, 1955 S 2 Sheets-Sheet 2 ZZi,35. 4.

M l8 TURNS IN DECOUPLING LOSS D) 3 A COUPLING LOSS (C) u |6 J 20 g D I210 a v C [c 1 INSERTION LOSS (1) FREQUENQY (MC) INVENTOR HERBERT J-CARLIN DECOUPLING (db) ORNEY United States Patent Q MULTIPLEX SIGNALLINGSYSTEM Herbert J. Carlin, Hicksville, N.Y., assignor to PolytechnicInstitute of Brooklyn, Brooklyn, N.Y., a corporation of New YorkApplication June 22, 1955, Serial No. 517,252

7 Claims. (Cl. 333-9) This invention relates to a multiplex signallingsystem for operating several transmitters and/or receivers on a singleantenna.

While the invention is especially useful in the ultra high-frequencyband, it is not limited to this frequency band.

A broad object of the invention is to devise a multiplexing system forthe simultaneous operation of several transmitters and receivers on asingle antenna with low frequency separation of the channels, lowinsertion loss, and good isolation between the different channels.

A more specific object of the invention is to devise a multiplexingcoupler formed of coaxial line elements, and being suitable foroperating several transmitters and receivers on a single antenna in theultra high-frequency band.

With the coupler described herein, for operation in the band of 225 to400 mc./s., it is possible to obtain satisfactory multiplexing with afrequency separation between adjacent channels of only 3 megacycles persecond, an average insertion loss in a four-coupler system ofapproximately 1 db per coupler, and an isolation between adjacentchannels of approximately 60 db.

The invention is illustrated in the accompanying drawing in which:

Figure l is a circuit diagram, partly in block fonn, showing a multiplexsignalling system according to the present invention and embodying threecouplers connected in cascade for operating two transmitters and tworeceivers from a single antenna;

Figure 2 is a circuit diagram illustrating the bridge arrangcn'ient ofthe elements embodied in each coupler;

Figure 3 shows a coaxial embodiment of the coupler acccrmng to thepresent invention, the coupler being shown in section along alongitudinal cutting plane; Figure 3a shows the equivalent circuit ofFigure 3; and

Figure 4 shows typical performance curves of the coupler.

Referring to Figure 1, three multiplexing couplers C1, C2 and C3 areconnected in cascade to a single antenna A connected to coupler C1. Atransmitter T1 operating at a frequency 7, is coupled to the antenna Athrough coupler C1 and a second transmitter T2 operating at a frequencyf is coupled to the antenna through another channel completed bycouplers C1 and C2. All three couplers complete another channel toreceiver R3 operating at a frequency f and the three couplers alsocomplete another channel to a tuned detector or. second receiver R4. L1,L2 and L3 represent balancing impedances connected to the three couplersfor purposes described later.

The circuit arrangement for each of the three couplers of Figure 1 isillustrated in Figure 2. As will be seen, the coupler is formed of asix-arm impedance bridge connected between the terminals a, b, c and d.The arm a-b comprises a series resonant circuit formed of an inductanceL and a condenser C having an impedance Z The arm ad is formed of abalancing load or variable impedance L. The arm cd is formed of aparallel tuned circuit (an anti-resonant circuit) consisting of aninductance L and a parallel condenser C forming an impedance Z,,. Thearm bc comprises an antenna or another coupler connected between theterminals b and c. The arm ac comprises transmitting or receivingequipment connected across terminals T, and the arm b--d comprisesanother coupler or an output device connected in the position N. LettersA, L, N and T in Figure 2 represent ports or terminals in the differentarms of the bridge. The bridge is balanced when the product of theimpedances in opposite arms are equal. That is Z Z -:Z =1

The impedance Z, is inverse to Z, at all frequencies over the workingband. Under these conditions power at port T is entirely decoupled fromarm N. At the same time it is easily seen that port A is also decoupledfrom port L. This decoupling is theoretically independent of frequencthough parasitic effects in the coaxial coupler prevent the fullrealization of this frequency-invariant performance. The decoupling isrelated to the Directivity of directional couplers and, in fact, theactual device is a coaxial directional coupler.

In addition it may be shown that the on resonance insertion loss from atransmitter to an antenna (T to- A) or the 01f resonance loss of acoupler (A to N) can be made small providing a high enough value of Qfor the resonant circuits of Fig. 2 is used, and provided that theresonant impedances of these tank circuits are suitably chosen. Ideally,Z should be low at resonance and Z high at that point. This permitscoupling of (T) to (A) via path abc. The oif-resonance value of Z mustincrease rapidly and Z decrease so that power from (A) bypasses (T) andgoes to (N) via path b-dc. With a finite Q, the resonance level of Zshould not be set too low, for the impedance level would then neverreach high enough off resonance values to produce sufficiently lowinsertion loss. Similarly, the resonant level of Z must not be permittedto be too high at resonance. A further problem is the loss of effectivedirectivity when the coupler is operated with a mismatched antenna; Thisis taken into account by the use of a variable impedance as a balancingnetwork L in arm ad of Fig. 2. The antenna balancing network not onlybalances out the mismatch due to the antenna but also cancels outparasitic effects in the coupler at whatever frequency the device istuned to.

Figure 3 shows a coaxial embodiment of the coupler bridge of Figures 1and 2. The series resonant circuit forming the arm a-b of the bridge isembodied in a coaxial structure extending horizontally in the upper partof Figure 3, and the parallel resonant circuit forming the arm c-d ofthe bridge is embodied in a coaxial structure extending verticallydownward from the first coaxial structure. The left end of the series.resonant unit is connected at port L to an adjustable balancingimpedance or load by a coaxial line having conductors marked ad, whilethe right end of this coaxial unit is connected at port N to the nextcoupler by a coaxial line with conductors marked bd. The two coaxialstructures are supported from a common base formed of block l which isbored horizontally for the series resonant cavity and is boredvertically for the parallel resonant cavity. The

- outer conductor of the series resonant structure is formed unit isformed of three sections, a short fixed section 2a connected with thecenter conductor (a) of the line at port L, a longer fixed section 2bspaced from section 2a, and a movable section 20 sliding within section2b and connected to the inner conductor (b) of the line at port N.

Mounted in the center of tubular section 2b is a center conductor 3, theleft end of which is supported by insulating plug 4 having threadedengagement with the interior of section 2a, the plug 4 being rotatableto control the extent to which the center conductor 3 extends out of thetubular section 2b, across the gap between tubular sections 2a2b, andinto the section 2a. A short-circuiting slider 5 is carried on the innerend-of tubular section 2c and serves to vary the length of the coaxialline formed of conductors 2b-3. Sliding of the outer conductor section1c also varies the length of the coaxial line formed by the concentricconductors 1alc and 2b2c. The length of the un-shorted portion ofconductor 3 should be about one-quarter of the operating wavelength. Asseen across the gap between conductor sections 2a and 2b, the innercoaxial structure presents a series-resonant unit, the approximateequivalent circuit of which is shown in Fig. 3a between terminals (a)and (b).

The parallel resonant coaxial structure is formed of sleeve extensions6a and 6b mounted upon the block '1 in alignment with the vertical boretherein. The inner conductor 7 is supported from the outer conductor 6by two oppositely extending arms 7a and 7b passing through the walls ofblock 1 and forming the outer conductors respectively of coaxial linescb and c-a connected to the coupler at ports A and T and leading to theantenna and transmitter respectively. The upper end of center conductor7 is suitably bored to permit the inner conductor (a) of line T to beconnected to the inner end of tubular section 2a, and the innerconductor b of coaxial line A to be connected to the inner end oftubular section 2b. The outer end of inner conductor 7 is formed of atubular section 7c, into which is slid-ably mounted another tubularsection 7d closed by a plug 7e. The outer end of sleeve 6b is shorted bya plug 6c, and adjustment of the telescoping sleeve 7d is accomplishedby means of an insulating rod 8 secured to the plug 7e and passingthrough a central hole in the plug 60. As seen from the upper end ofcenter conductor 7, the vertical coaxial structure acts as aparallel-resonant unit, the approximate equivalent circuit of which isshown in Fig. 3a connected between terminals (0) and (d). Varying thelength of the inner conductor 7c7d varies the tuning of the parallelresonant unit. The resonant frequency of the unit depends on the lengthof the adjustable section '7c'7d,and also upon the length of the arms 7aand 7b within the block 1, and the. lengths of these arms may be variedby suitable means, if desired, such as the level control sleeve 9 shownon arm 7a, which may be duplicated on arm 7b if necessary, as shown at9a.

The approximate equivalent circuit of Figure 3 is shown in Figure 3a.The parallel combination L C in Figure 3a represents the near resonantline formed of conductors 2b3, while the condenser C represents theopen-circuited line section formed of conductors 241-3. By varying theaxial position of the conductor 3 with respect to the coupling slot a-bin Figure 3 (by adjusting plug 4), the position of the coupling slotalong the resonant line is thereby varied, giving the elfect of animpedance transformation by which the resonant impedance level may becontrolled.

The anti-resonant unit of Figure 3 consists of an opencircuited lengthof line in shunt witha short length of short-circuited line formed ofinductive stubs 7a and 7b which produce an effect of animpedancetransformer. By adjusting the lengths of the inductive stubs 7a and 7b'by the sleeves 9 and 9a, the impedance level of. the anti-resonant unitis varied. Theinductance-addedby 4 arms 7a and 7b is shown at L, inFigure 3a. The third reactive element added to each of the cavityequivalent circuits causes a second resonant point which is locatedclose to the first.

The arrangement shown in Figure 3 provides for various junction pointsto be brought out and made accessible for external coaxial lineconnections, such as those pro vided by coaxial stubs 7a and 7b whichprovide space for an inner coaxial conductor as explained above. Also,the various coaxial lines are arranged within the main junction block '1insuch manner that a minimum of space is required for the innerconnection of the-lines, and so that the points shown to'be directlyconnected together in Figure 3a are actually so connected in the coaxialembodiment. The dimensions of the junction block should be as small aspossible with respect to the operating Wavelength.

Simply by way of illustration, the approximate dimensions will be givenfor a coupler designed for operation over a freqency band from 225 to400 megacycles per second.

Referring to Figure 3, the two coaxial lines at ports'L and N connectingthe coupler to the adjustable load or impedance and to the next couplerare of the same size, the effective diameters of the inner and outerconductors of these lines being 0.356 and 0.811 inch, respectively. Thecoaxial lines at the ports A and T connecting the coupler to the antennaand to the transmitter respectively are of the same size, and theeffective diameters of the inner and outer conductors of these lines are0.175 and 0.285 inch, respectively. These lines enter the block 1through bores of a diameter of 0.625 inch. The horizontal and verticalbores through block 1 for the two coaxial resonant units are both of adiameter of 2 inches. The inside diameter of sleeve 1b is approximately1.3 inches, and the inside diameter of sleeve in is 3 inches. Tubularconductor section 2a has an inside diameter of 0.811 and an outsidediameter of 0.875 inch, respectively, and the end of tubular conductorsection 2b located within the block 1 is of the same dimension, whilethe section of this tubular conductor located within the sleeve M has aninside diameter of 1.265 and an outside diameter of 1.315 inches,respectively. The section of the inner conductor 3 located within thesleeve 1a has an outside di ameter of 0.55 inch, while the section ofthis conductor located within theb lock 1 has an outside diameter of0.356 inch. The length of the gap between tubular conductor sections 2aand 2b is 0.25 inch. Tubular section 1b has a length of approximately 2inches. The horizon tal bore in blockl has a length of approximately 5inches, and the length of tubular section 1a is approximately 13 inches.Tubular section 10 sliding within section 1a has a length ofapproximately 10 inches. Section 20, including the short-circuitingslider 5, has a length about the same as section 10. Tubular sections 10and 20 have a range of anal movement of about 9 inches. The centerconductor 3 has an overall length of about 15 inches, and its effectivelengthis varied by slider 5 from about 4 inches to about 13 inches. Thespacing between the center of the horizontal bore in block 1 and theaxis of the bores receiving the lines 7a and 7b is approximately 2.25inches, and the distance between the axis of the lines 70 and 7b to theend of the coaxial line6a-6b is approximately 15 /2 inches. The sleeve70 has an outside diameter of 1.097 inches and the sleeve 6a has aninside diameter of 2.5 inches. Themovable sleeve 70! has anoutside/diameter of 1.02 inches, and the sleeve 6b has an insidediameter of 2.37 inches. This sleeve has a range of axial movement ofabout 4 inches.

A typical three-coupler system is shown diagrammatically in Figure 1. Tooperate this system the first transmitting coupler C1 has its tuningcavities set at the desired transmitter frequency f There is aprecalibrated frequency scale on the coupler, and the cavity plungersare Set m dssfieifi auens m r n Th t n g piungers of the other couplersare set at the assigned frequencies of f and f respectively. There isalso a tuning screw which adjusts the resonant impedance level of theresonant cavities when required.

:The final adjustment of each coupler is made with the antenna balancingnetwork associated with the coupler. A tunable detector R4 at the end ofthe coupler chain or array is employed as a null indicator in securingproper adjustment of the couplers. This is set for the transmitterfrequency assigned to the coupler being adjusted and the balancingnetwork is then tuned for a null indication on the detector. Thus,indicator R4 would first be tuned for f and then the balancing net workL1 of the transmitting coupler C1 set for a null indication by thedetector. Next the detector R4 would be set for E, and the balancingnetwork L2 of the coupler C2 (presuming its cavities have already beenset at the precalibrated scale indication for f adjusted for a nullindication at R4. Note that since the next coupler port of coupler f hasbeen decoupled from the transmitter port T1, the adjustment of couplerC2 has no elfect whatever on the previously tuned setting of coupler C1,and, in fact, the adjustment of each coupler in the chain is entirelyindependent of all the others.

a :Typical performance curves of the coupler described above, when tunedat 300 mc./ sec. and operating with a balancing network, are shown inFig. 4. In this figure, the insertion loss curve I is the coupling fromA to N (values of I at other than the resonant frequency are the olfresonance insertion losses). The coupling loss curve C shows the lossmeasured from A to T (the resonant value of C is the on resonance loss),and the curve D shows decoupling loss (synonymous with isolation) asmeasured from T to N. The letters refer to the port designations ofantenna (A), transmitter or receiver (R), or next coupler (N) of Figs. 2to 3a. Performance data of a 4 coupler system (a total of 4equipments-transmitters and/or receivers with a matched load on port Nof the last coupler) based on curves of this type, are given In theabove table the average loss per coupler is the total loss to the lastcoupler set for the frequency shown divided by 4.

'It should be emphasized that the freedom from interaction eifectsbetween units of the coupler system is almost entirely independent ofthe closeness of frequency spacing of adjacent equipments; If a UHFreceiver of kc. bandwidth is used, it can operate even closer than 1 me.to an adjacent high power transmitter and the residual transmittersignal will still be low enough in level to be adequately discriminatedagainst by the receiver selectively. The main limitation on closeness offrequency spacing is almost entirely a matter of tolerable insertionlosses, and if these are to be of the order of ldb average, then aminimum spacing of about 3 mc./sec. is required. If higher losses arepermissible (of the order of 3 db per coupler), 1 mc./ sec. separationsare entirely feasible.

One advantage of the coupler bridge of the invention is that theadjustable impedances L1, L2, etc. provide means for compensating for amismatched antenna. This is easily understood by reference to Figure 2.If the load in that figure is an adjustable impedance then it may be setto balance the bridge, i.e. produce a null in arm N, when the antennaarm A presents an impedance other than a purely resistive impedance. Theadjustable balancing impedance should be capable of producing eithercapacitive or inductive reactance of adjustable value.

I claim:

1. A multiplexing signalling system comprising a transmission line, aplurality of parallel-resonant circuits connected in cascade in saidtransmission line at .a plurality of station locations, said circuitsbeing tuned to different frequencies, a signalling device and abalancing impedance connected in series relation in a path shunting eachparallel-resonant circuit, and a series-resonant circuit connectedacross said transmission line at each station and being tuned to thesame frequency as the parallel-resonant circuit at the same station, oneterminal of each series-resonant circuit being connected to themid-point of the path shunting the parallel-resonant circuit at the samestation, the parallel resonant circuit at each station location havingan impedance which is the inverse of the impedance of theseries-resonant circuit at the same station.

2. A multiplex signalling system comprising .a plurality of couplernetworks connected in a cascade array, each coupler comprising fourterminals a, b, c, and d, a signalling device for each coupler connectedacross terminals a and c and operating at an assigned frequencydifferent from frequencies assigned to other devices, a balancingimpedance for each coupler connected between terminals a and d, aparallel-resonant circuit connected between terminals 0 and d and beingtuned to said assigned frequency, a series-resonant circuit connectedbetween terminals a and b and being tuned to said assigned frequency,and connections from terminals b and d of one coupler to terminals b andc of the adjacent coupler in said cascade array, the parallel resonantcircuit in each network having an impedance which is the inverse of theimpedance of the series-resonant circuit in the same network.

3. A multiplex signalling system comprising a plurality of couplernetworks connected in a cascade array, each coupler comprising fourterminals a, b, c, and d, a signalling device for each coupler connectedacross terminals a and c and operating at an assigned frequencydifferent from frequencies assigned to other devices, a balancingimpedance for each coupler connected between terminals a and d, aparallel-resonant circuit connected between terminals 0 and d and beingtuned to said given frequency, a series-resonant circuit connectedbetween terminals a and b and being tuned to said given frequency,connections from terminals b and d of one coupler to terminals b and cof the adjacent coupler in said cascade arrangement in said cascadearray, a multi-channel circuit for the transmission of a plurality ofsaid frequencies connected between terminals b and c of the coupler atone end of said array, and a signal device operating at one of saidassigned frequencies connected to terminals b and c of the coupler atthe other end of said cascade array, the parallel resonant circuit ineach network having an impedance which is the inverse of the impedanceof the series-resonant circuit in the same network.

4. A multiplexing coupler comprising two sections of coaxial cablearranged in spaced axial alignment and having load terminals and couplerterminals respectively, a first tubular conductor connecting the outerconductors of said aligned cable sections and being of [larger diameterthan the outer conductors of said cable sections, a second tubularconductor mounted concentrically within said first tubular conductor andbeing formed of two sections separated by a gap, one section beingrelatively short and being connected to the inner conductor of one cablesection and the other section being relatively long and connected to theinner conductor of the other cable section, a center conductor mountedwithin said long section of said second tubular conductor and beingslid-- 7' able axially thereof to extend out of said [long sectionacross said; gap and into said short section, means for varying thelength of the coaxial line formed between said first and second tubularconductors, means for varying the length of the coaxial line formedbetween said long section of tubular conductor and said inner conductor,a tunable coaxial line coupledto said first tubular conductor through anopening formed in the wall thereof adjacent said gap and embodying aparallel-resonant circuit tunable over a band of frequencies, a coaxialline for connection to signalling apparatus and having an innerconductor connected to the end of said short section adjacent said gap,and another coaxial line for connection to a signal channel and havingan inner conductor connected to the end of said long section adjacentsaid gap.

5. A multiplexing coupler comprising two sections of coaxial cablearranged in spaced axial alignment and having load terminals and couplerterminals respectively, a first tubular conductor connecting the outerconductors of said aligned cable sections and being of larger diameterthan the outer conductors of said cable sections, a second tubularconductor mounted concentrically'within said first tubular conductor andbeing formed of two sections separated by a gap, one section beingrelatively short and being connected to theinner conductor of one cablesection and the other section being relatively long and connected to theinner conductor of the other cable section, a center conductor mountedwithin said second tubular conductor and being slidable axially thereofto extend out of said long section across said gap and into said shortsection, means for varying the'length of the coaxial line formed betweensaid first and second tubular conductors, means for varying the lengthof the coaxial line formed between said long section of tubularconductor and said inner conductor, a third.tubular.conductor coupled tosaid first tubular conductor through an opening formed in the wallthereof adjacent said gap and being closed at its outer end, a centerconductor of adjustable length mounted within said third tubularconductor and having a front ,end portion terminating in said opening insaid first tubular conductor, said last mentioned center conductor beingsupported by a pair of oppositely extending arms spaced from the frontend thereof and passing through openings in said third tubularconductor, a coaxial line for connection to signalling apparatus andembodied in one of said arms and having an inner conductor extendingthrough aborein said front end section of said center conductor andbeing connected to the end of said short section adjacent said gap, andanother coaxial line for connection to a signal channel and embodied inthe other arm of said center conductor and having an inner conductorpassing through a bore in said front end section of said centerconductor and connected to the end of said long section adjacent said 6.A multiplexing coupler comprising two sections of coaxial cable arrangedin spaced axial alignment and having load terminals and couplerterminals respectively, a first tubular conductor connecting the outerconductors of said aligned cable sections, a second tubular conductormounted concentrically within said first tubular conductor and beingformed of two sections separated by a gap, one section being relativelyshort and being connected to the inner conductor of one cable section gi and the other section being relatively long andc'o'nnected to theinner conductor of the'other cable section, a center conductor.mountedwithin said long section of said second tubular conductor and havingoneaend shorted to .said second tubular conductor and the other endterminating adjacent said .gap-,..said secon'd tubular conductor andsaid center conductor forming a seriesresonant circuit across saidgap;atunable coaxial line coupled to said first tubular conductor through anopening formed in the wall thereof adjacent saidgap and embodying aparallel resonant circuit tuned to the same frequency as saidseriesfresonant circuit; a coaxial line for connection to signallingapparatus and having an inner conductor connected to the end of saidshort sec tion adjacent said gap; and another coaxial line forconnection to a signal channel and .having an 'inner conductor connectedto the end of said ;long section adjacent said gap.

7. A multiplexing c 'lupler comprising two sections of coaxial .cablearranged in spacedaxial alignment and having load terminals and couplerterminals respectively, a first tu bular conductor connecting theouter'conductors of said aligned cable sections, a second tubularconductor mounted concentrically Within said first tubular conductor andbeing formed of two sections separated by agap, one sectionbeingrelatively short and being connected to the inner conductor of one cablesection and the other section being relatively long and connected to theinner conductor of .the other cable section, a center conductor mountedwithin said second tubular conductor and having one end shorted to saidsecond tubular conductor and the other end terminating ad jacent saidgap, said second tubular conductor and said center conductorfonning aseries-resonant circuit across said lgap; a tunable coaxial line coupledto said first tubular conductor and forming a parallel-resonant circuittuned to the same frequency as said series-resonant circuit, saidcoaxial line'comprising a-third tubularconductor coupled to said firsttubular conductor through an opening in the wall thereof adjacent saidgap, asceond center conductor mounted within said third tubularconductor and having a front end portion terminating in said opening insaid first tubular conductor, said second center conductor beingsupported by a pair of oppositely extending arms spaced from the frontend thereof and passing through openings in said third tubularconductor; a coaxial line for connection to signalling apparatus andembodied in one of said arms and having an inner conductorextending-through a bore -in said front end section of said centerconductor, and bein connected to the end of saidshortsection adjacentsaid gap, and another coaxial line for connection to a signal channeland embodied in the other arm of'said center conductor and having aninner conductor passingthrough a bore in said front .end section of saidcenter conductor and connected to the end of said long section'adjacentsaid gap.

References Cited in the file of this patent UNITED STATES PATENTS2,495,589 Masters Jan. '24, 1950 2,713,152 Brown July '12, 19552,779,000 Sosin Jan.-22, 1957

